In the development of X-ray film handling systems, the equipment has been provided which allows the automatic loading and unloading of X-ray film into a cassette under daylight conditions. Such equipment frees the operator from the need to load and unload the film to and from a cassette in a dark room. Exemplary of such an automatic system is that disclosed in Schmidt U.S. Pat. No. Re. 28,438. This reissue patent describes an X-ray film package for use with the daylight loading cassette of the type disclosed in Schmidt U.S. Pat. Nos. 3,784,835 and 3,870,889. Schmidt U.S. Pat. No. 3,715,087 describes a device used for unloading cassettes of the last mentioned patents under daylight conditions.
In operation, a film sheet is automatically loaded into the cassette which is operative to hold the film between two opposed substantially parallel X-ray intensifying screens during a period of patient exposure. Following exposure, the cassette is unloaded under the influence of gravity using a device as disclosed in the last mentioned patent which is adapted to release one of the two cassette plates supporting the intensifying screens to thereby relieve the holding pressure from the film sheet and allow it to drop under the influence of gravity into a light-tight container.
In designing and constructing X-ray cassettes, an important consideration is contact between a film sheet and the intensifying screens. Not only should such contact be intimate, but it should be uniform over the full area of the intensifying screens. One of the problems encountered as the X-ray cassettes are made larger, is that as the two intensifying screens are brought into contact with the film sheet placed thereinbetween, air trapped between the screen and the film thereby reducing contact and resulting in a radiographic image of poorer resolution. The problem is particularly significant when high resolution smooth surface screens are used. In order to eliminate this problem, it has been proposed to use a mounting plate for the intensifying screens which may be curved to assume either a cylindrical or circular shape. While this solution has been practiced in cassettes designed to open somewhat like a book to allow insertion of a film sheet between the two intensifying screens, such solution has not been practical in cassettes of the type disclosed above because the daylight loading cassette thickness is fixed to a small dimension corresponding to that of a closed book-type cassette. Within this limited dimension, not only must all components of the cassette be included, but, in addition, there must be allowed sufficient room for one of the screens to be able to move away from the other in order to allow the film sheet to be inserted and removed therefrom. Thus, employment of a curved intensifying screen becomes impractical for the daylight loading cassette.
A second problem typical of the aforementioned Schmidt daylight loading cassette is the time required for a film sheet to be discharged therefrom once the cassette has been placed on the film release equipment. Film discharge is obtained by releasing the pressure applied to one of the intensifying screens which forces the screen against the film sheet. If all the air has been properly excluded from between the intensifying screens and the film sheet during the loading operation. before the film can drop from the cassette, sufficient time must elapse for air to seep back between the film sheet and the screens to release the film sheet from the screens. For a variety of reasons, it is desirable that the discharge time of a film sheet from such a cassette should not exceed about three seconds.
Thus, there is still need for the development of a daylight X-ray cassette of the type discussed above in which good film screen contact is available without any increase in the thickness of the cassette and without unduly increasing the release time of a film sheet therefrom.